The invention is generally related to power supplies and power converters, and in particular, to full bridge power converters incorporating resonant switching to minimize switching loss.
Full bridge power converters are typically used in power supplies to convert a direct current (DC) power signal at one voltage to a DC power signal at another voltage. For example, in large multi-user computers, full bridge power converters are used to convert relatively high input power signals, e.g., about 400 VDC, to low power signals suitable for diving integrated circuitry, e.g., less than 5 VDC.
In one conventional design of a full bridge power converter, a full bridge transistor topology is formed by a set of four transistors, typically metal oxide silicon field effect transistors (MOSFET""s). The bridge transistors are broken into two pairs, also referred to as xe2x80x9clegsxe2x80x9d, with the transistors in each pair coupled in series, and the pairs coupled in parallel between the positive and negative input terminals (also referred to as high and low rails) of the converter. Outputs are formed by the common nodes between the transistors in each series pair. A capacitor is coupled in parallel with the full bridge, and the primary winding of a transformer is coupled between the outputs defined at the common nodes between the transistors in each series pair.
A full bridge power converter operates by supplying phase-shifted pulsed control signals to the transistors to provide a pulse width modulated (PWM) output signal at the secondary output of the transformer. Power is supplied to the transformer whenever the transistors at opposite corners of the bridge are simultaneously conducting, such that current flows from the positive input terminal, through the transistor in one leg of the bridge, through the transformer, and through the opposite transistor in the other leg back to the negative input terminal. Each transistor is typically supplied with a 50% duty cycle pulse, with the transistors in each series pair out of phase with one another such that at most one transistor, but not both, is on at any given time. The relative phasings between the series pair signals control the amount of time (or width) of the PWM output signal generated by the transformer. Using conventional rectification and filtering, an output signal with a desired DC voltage may be generated from the PWM signal.
An important concern with any power supply circuitry is efficiency, i.e., the percentage of input energy that is output as useful energy from the circuitry. Efficient circuitry consumes less power and generates less heat, which in turn enhances reliability. Another concern is electromagnetic interference (EMI), which can disturb the operation of adjoining circuitry absent appropriate shielding.
In some full bridge power converter designs, a significant contributor to both inefficiency and EMI emissions are the switching losses associated with the bridge transistors. For a MOSFET, for example, switching loss is a function of the voltage drop between the source and drain terminals of the MOSFET when the switch occurs. Switching losses increase with frequency; however, higher frequency reduces the required size of the magnetics (e.g., the transformer windings) in the circuit. Thus, a tradeoff has conventionally existed between operational frequency and switching losses.
To address these concerns, some full bridge power converter designs incorporate xe2x80x9cresonant switchingxe2x80x9d to decrease the switching losses in the bridge transistors. Resonant switching is enabled by storing energy in the bridge and delaying the switching of one transistor in a leg to an on state after the other transistor in the leg is switched off. Doing so enables the stored energy in the bridge to charge the parasitic capacitance in the transistor that has been switched off to pull the voltage at the common node toward that at the input terminal to which the other transistor is coupled, thereby decreasing or eliminating any voltage drop across the latter transistor. Reducing the voltaic drop across the latter transistor likewise reduces switching losses when the transistor is turned on.
In practice, the control signals applied to one leg of the bridge (referred to as the xe2x80x9cleftxe2x80x9d leg) lead the control signals applied to the other leg of the bridge (referred to as the xe2x80x9crightxe2x80x9d leg) such that transitions that occur on the left leg initiate a reversal in the direction of current flowing through the primary winding. Put another way, the left leg transitions typically occur at the start of the clock cycle, while the right leg transitions are controlled via a ramp and feedback error signal to modulate the duty cycle.
It has been found that resonant switching is more difficult when dealing with transitions in the xe2x80x9cleft legxe2x80x9d transistors than those in the xe2x80x9cright legxe2x80x9d transistors. Typically, right leg transitions are adequately charged by the energy storage capability of the primary winding of the transformer (specifically the magnetizing inductance and/or the leakage inductance inherent in the primary winding), such that the winding functions as a current source. At left leg transitions, however, the energy stored in the primary winding is transferred to the transistor capacitance, discharging the capacitance to zero volts. This is a xe2x80x9ctruexe2x80x9d resonant condition compared to the right leg case where the primary winding energy remains fairly constant through the transition (i.e., a current source). If the inductive energy is equal to or greater than the capacitive energy, then ZVS (zero voltage switching) is assured.
In many instances, particularly under light load where energy storage in the winding is the lowest, the inductance in the primary winding is not sufficient to fully enable resonant switching in the left leg. As a consequence, some full bridge power converter designs incorporate a shim inductor in series with the transformer primary winding between the bridge transistor legs. However, the use of a shim presents a number of drawbacks. For example, the shim must be sized to carry all of the primary current and reflected load current, thus increasing the amount of magnetics in the circuit. The additional inductance also limits the slew rate of the primary current, which increases the time period required to swing the primary current between positive and negative maximums, and thereby decreases the available duty cycle at the secondary output.
Therefore, a significant need continues to exist in the art for a manner of decreasing switching losses and thereby increasing the efficiency of a full bridge power converter, in particular, to address switching losses associated with left leg transitions in the same.
The invention addresses these and other problems associated with the prior art by coupling an energy storage circuit to at least one leg of a full bridge transistor topology to facilitate resonant switching within that leg. Typically, the energy storage circuit includes an inductor coupled between the common node between the transistors in a leg and a reference voltage that is intermediate the relative voltages at which the inputs of the full bridge transistor topology are biased. The voltage drop across the inductor from the reference voltage to the common node permits energy stored in the energy storage circuit to charge the parasitic capacitance of a transistor that has been shut off during a transition, and thereby decrease the voltage drop across the complementary transistor to be turned on during the same transition. As a result, switching losses are reduced, or eliminated, in the transistor being turned on, and overall efficiency of the circuit arrangement is improved. Moreover, while an energy storage circuit may be used to enhance resonant switching in either leg of a full bridge transistor topology, when used in connection with the left leg of the topology (where transitions result in a reversal of the direction of current flow), the additional energy supplied by the energy storage circuit is often sufficient to provide the energy necessary to fully discharge the transistor capacitance, resulting in lossless switching that is independent of load current.
In some implementations, it may also be desirable to vary the energy storage of the energy storage circuit responsive to load (e.g., by placing a switch in series with an inductor), such that the amount of energy supplied by the energy storage circuit is inversely proportional to the amount of load experienced by the circuit. Doing so often improves the overall efficiency of a full bridge power converter across a wider range of loads.
These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention.